A typical embedded system (hardware and software which forms a component of some larger system and which is expected to function without human intervention) includes at least one integrated circuit memory such as a static random access memory (SRAM). A disc drive, utilized for data storage in modern electronic products ranging from digital cameras to computer systems and networks, is one such embedded system that includes integrated circuit memories.
Typically, a disc drive includes a mechanical portion, or head disc assembly (HDA), and electronics in the form of a printed circuit board assembly (PCBA), mounted to an outer surface of the HDA. The PCBA controls HDA functions and provides an interface between the disc drive and its host.
Generally, a HDA comprises moving parts such as one or more magnetic discs affixed to a spindle motor assembly for rotation at a relatively constant speed, an actuator assembly supporting an array of read/write heads that traverse generally concentric data tracks radially spaced across the disc surfaces, and a voice coil motor (VCM) providing rotational motion to the actuator assembly.
A disc drive PCBA includes a microcontroller and at least one integrated circuit memory, such as a SRAM, that stores program instructions and other data. The program instructions are executed by the microcontroller to thereby provide communication between the host computer and the HDA of the disc drive.
It is well known that the logic state of a SRAM bit cell can change if an energetic particle such as an alpha (α) particle strikes the cell. Such “soft-error” or “single event upset” (SEU) effects can occur if an α-particle strike generates, for a sufficient duration, a charge having a magnitude exceeding the critical charge in one of the cell's storage nodes (i.e. the minimum electrical charge needed to change the cell's logic state). Since trace amounts of α-particle emitting constituents are unavoidably found in semiconductor packaging, silicon wafers, and especially in the naturally occurring radioactive lead (Pb) used in “flip-chip” packaging, any modern embedded system having a relatively large amount of integrated circuit memory is potentially susceptible to a significant soft-error rate.
The prior art has evolved various techniques for minimizing the susceptibility of integrated circuit memories, of the type included in embedded systems, to SEU effects. One approach, is to increase the memory cell's charging time constant and thereby decrease the cell's susceptibility to SEU effects. For example, an RC circuit can be interconnected between the two cross-coupled inverter stages in a typical six-transistor SRAM cell. An undesirable side effect of this approach is that the added capacitance increases the cell's write time. Another prior art approach is to add a small amount of capacitance and/or resistance to each of the cell's storage nodes. However, this significantly increases the cell's integrated circuit surface area, which is undesirable. In general, the prior art solutions, which have primarily focused on decreasing individual cell's susceptibility to SEU effects, have shortcomings such as high complexity and/or relatively high cost.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.